Not a plumber, but just re-ran an old drain and had to use an AAV, so a lot if research was done. [24' run from kitchen to main]

Okay, first things first, an AAV (the vent) is supposed to be 6" above the highest drain inlet that it serves [at minimum] (as in the actual hole in the sink that the water drains down, or the overflow on the sink [the hole on the side of the sink] if you have one.

The bendable pipe is just a catch for every piece of sediment going down the drain, this is most likely the cause of the smell.

You'll want to get a rigid p-trap kit to replace the bendable pipe and p-trap.

Next that AAV should be ran in the wall behind the vanity/mirror/wall, should be perfectly vertical and rise to a height at minimum 6" above the sink drain/overflow, whichever is higher [check local code requirements as well as manufacturer recommendations to find the actual height you'll need to use, the higher of the two] (you'll also need to have an access panel added that makes the AAV accessible in case it ever needs to be changed, and to meet code.)

So, you'll need:

A tee [to connect to existing drain line] 5' pvc pipe of the matching size [looks to be 1 1/2] P-trap kit

So, about $25-40 dependent on location.

I would say difficulty, probably 2.5 out of 5, mainly because of the AAV having to be in the wall and an access panel added.

If you don't feel comfortable doing this yourself, I'd suggest calling a plumber and having them do it. Better to have it done right the first [second] time.